1. Field of the Invention
The present invention relates to a resin-filled ferrite carrier for an electrophotographic developer used in a two-component electrophotographic developer used in copiers, printers and the like, a production method thereof and an electrophotographic developer using this ferrite carrier. More specifically, the present invention relates to a resin-filled ferrite carrier for an electrophotographic developer having a lightened true density and a lengthened life, and which can ensure the stability of the charge properties and which is free from image defects such as white spots, a production method thereof and an electrophotographic developer using this ferrite carrier.
2. Description of the Related Art
Electrophotographic developing methods develop by adhering toner particles in a developer to an electrostatic latent image which is formed on a photoreceptor. The developer used in such methods can be classified as either being a two-component developer composed of toner particles and carrier particles, or a one-component developer which only uses toner particles.
Among such developers, as the developing method using a two-component developer composed of toner particles and carrier particles, a cascade method or the like has long been employed. However, currently magnetic brush methods using a magnet roll have become mainstream.
In a two-component developer, carrier particles act as a carrying substance for imparting the desired charge to the toner particles and transporting the thus-imparted toner particles with a charge to the surface of the photoreceptor to form a toner image on the photoreceptor by stirring the carrier particles with the toner particles in a developing box which is filled with the developer. Carrier particles remaining on the developing roll which supports the magnets return back into the developing box from this developing roll, and are then mixed and stirred with new toner particles for reuse over a certain time period.
Unlike one-component developers, in two-component developers the carrier particles are mixed and stirred with the toner particles to charge the toner particles. The carrier particles also have a transporting function and are easily controlled when designing the developer. Therefore, two-component developers are suitable for full color developing apparatuses in which high image quality is demanded and for apparatuses performing high-speed printing in which the reliability and durability of image sustainability are demanded.
In two-component developers which are used in such a manner, the image properties, such as image density, fogging, white spots, gradation and resolution, need to exhibit a certain value from the initial stage. Furthermore, these properties must not change during printing and have to be stably maintained. To stably maintain these properties, it is necessary for the properties of the carrier particles in the two-component developer to be stable.
Conventionally, an iron powder carrier, such as iron powder covered with an oxide coating on its surface or iron powder coated with a resin on its surface, has been used for the carrier particles forming a two-component developer. These iron powder carriers have high magnetization as well as high conductance, and thus have the advantage that an image with good reproducibility of the solid portions can be easily obtained.
However, the true specific gravity of such an iron powder carrier is about 7.8, which is heavy, and its magnetization is too high. As a consequence, the toner constituent component tends to fuse to the surface of the iron powder carrier, so-called “toner spent”, from the stirring and mixing with the toner particles in the developing box. Due to the occurrence of toner spent, the effective carrier surface area decreases, whereby the frictional chargeability with the toner particles tends to deteriorate.
With a resin-coated iron powder carrier, the resin on the surface may peel away due to stress during use, causing charge to leak as a result of the high conductance, low dielectric breakdown voltage core material (iron powder) being exposed. The electrostatic latent image formed on the photoreceptor breaks down as a result of such charge leakage, thus causing brush strokes or the like to occur on the solid portions, which makes it difficult to obtain a uniform image. For these reasons, iron powder carriers, such as an oxide-coated iron powder or a resin-coated iron powder, are currently no longer used.
Recently, instead of iron powder carriers, resin-coated ferrite carriers coated with a resin on their surface are often used which use a ferrite core material having a light true specific gravity of about 5.0 and a low magnetization, whereby developer life has become dramatically longer.
However, in recent years the workplace has become more networked, evolving from an era of single-function copiers to multifunction devices. In addition, the type of service provided has shifted from a system wherein a contracted repair worker carries out regular maintenance and replaces the developer and other parts to a maintenance-free system. Further, demands from the market for even longer developer life are becoming much greater.
Further, full color images are now standard in the workplace, so that there is an increasing demand for higher quality images. Toner particle size is also decreasing in order to obtain higher resolution.
In response to these demands, the carrier particle size is also shifting towards a smaller particle size having a higher specific surface area, as it is necessary for the desired charge to be quickly charged onto the toner. If the overall particle size distribution moves to a smaller particle size, the particles on the finer powder size, especially, are more likely to scatter or adhere to the photoreceptor, so-called “carrier adhesion”. As a result, critical image defects such as white out are more easily induced. Therefore, small particle size carriers must be controlled to have an even narrower particle size distribution width.
In view of these circumstances, many proposals have been made concerning magnetic powder-dispersed carriers in which fine, magnetic microparticles are dispersed in a resin to extend developer life by making the carrier particles lighter.
Such a magnetic powder-dispersed carrier can reduce true density by reducing the amount of magnetic microparticles, thus reducing the stress from stirring. As a result, chipping or peeling of the coating can be prevented, whereby stable image properties for a long period of time can be obtained.
However, because a binder resin covers the magnetic microparticles, the magnetic powder-dispersed carrier has a high carrier resistance. Thus, there is the drawback that it is difficult to obtain sufficient image density.
In addition, since the magnetic microparticles are hardened by the binder resin, the magnetic powder-dispersed carrier has also had the drawbacks that the magnetic microparticles detach due to stirring stress or from shocks in the developing apparatus, and that the carrier particles themselves split, possibly as a result of having inferior mechanical strength as compared with the conventionally-used iron powder carrier or a ferrite carrier. The detached magnetic microparticles or split carrier particles adhere to the photoreceptor, thereby becoming a factor in causing image defects.
Further, a magnetic powder-dispersed carrier has the drawback that since fine magnetic microparticles are used, remnant magnetization and coercive force increase, so that the fluidity of the developer deteriorates. Especially when a magnetic brush is formed on a magnet roll, the bristles of the magnetic brush stiffen due to the presence of remnant magnetization and coercive force, which makes it difficult to obtain high image quality. There is also the problem that even when the carrier leaves the magnet roll, because the carrier magnetic agglomerations do not come unloose and the carrier cannot be rapidly mixed with the supplied toner, the rise in the charge amount is poor, which causes image defects such as toner scattering and fogging.
A resin-filled carrier in which the voids in a porous carrier core material are filled with a resin has been proposed as a replacement for magnetic powder-dispersed carriers. For example, Japanese Patent Laid-Open No. 11-295933 describes a carrier which comprises a polymer contained in the pores of cores, and a coating which covers the cores. These resin-filled carriers enable a carrier to be obtained having few shocks, a desired fluidity, a broad range of frictional charge values, a desired conductance and a volume average particle size that is within a certain range.
Japanese Patent Laid-Open No. 11-295933 describes that various suitable porous solid core carrier substances, such as a known porous core, may be used as the core material.
However, as is described in the examples of Japanese Patent Laid-Open No. 11-295933, for a porosity of about 1,600 cm2/g in BET surface area, a sufficient reduction in the specific gravity is not achieved even by filling with a resin.
If a large amount of resin is filled into a core material, the resin which could not be filled remains by itself without closely adhering to the core material. In such a state, the left-over resin floats in the carrier, causing a large amount of agglomerates to form among the particles, whereby fluidity deteriorates. When agglomerates break apart during use, charge properties fluctuate greatly, making it difficult to obtain stable properties.
Further, in Japanese Patent Laid-Open No. 11-295933, a porous core is used, and the total content of the resin filled in the cores and the resin which coats the surface of the cores is preferably about 0.5 to 10% by weight of the carrier. In the examples of Japanese Patent Laid-Open No. 11-295933, the greatest total content of the resins does not even reach 6% by weight of the carrier. With such a small amount of resin, the desired low specific gravity cannot be realized, meaning that a performance that is merely approximate to that of the conventionally used resin-coated carrier is obtained.
Additionally, the carrier described in Japanese Patent Laid-Open No. 11-295933 not only has a core material which is insufficiently porous, but the amount of filled resin is also insufficient, and thus a resin-filled carrier having a three-dimensional layer structure in which a resin layer and a ferrite layer are alternately present cannot be obtained. The present inventors discovered that a resin-filled carrier having a three-dimensional layer structure in which a resin layer and a ferrite layer are alternately present a plurality of times can be obtained by filling resin into the voids of a porous ferrite core material wherein the voids are continuous from the surface through to the core material interior. The term “three-dimensional layer structure” as used here refers to, in a carrier particle cross section, a structure in which a plurality of resin layers and ferrite layers alternate with each other from one end to the other along a straight line (diameter) drawn passing through the center of the particle. The present inventors discovered that by forming such a three-dimensional layer structure, due to the retention of a capacitor-type nature, the structure has excellent charging capability and stability, yet has a high strength as compared to a magnetic powder-dispersed carrier. As a result, the structure has the advantage of not splitting, deforming or melting from heat or shocks.
While the carrier disclosed in Japanese Patent Laid-Open No. 11-295933 fills a resin or a fine powder consisting of an electrical insulating resin, essentially the way in which this is carried out is to merely increase the amount of resin in a carrier having a surface of a conventionally-known core coated by a resin, and just a tiny amount of this seeps into the voids. Charging capability and stability are not at a satisfactory level.
Japanese Patent Laid-Open No. 2006-337579 proposes a resin-filled carrier wherein a resin is filled in a ferrite core material having a void fraction of 10 to 60%. In Japanese Patent Laid-Open No. 2006-337579, because the carrier is filled with a resin, it has a lighter true density, can achieve a longer life and has excellent fluidity. Further, depending on the selection of the resin which is filled, it is easy to control the amount of charge or the like, yet the carrier is stronger than a magnetic powder-dispersed carrier, so that there is no splitting, deforming or melting from heat or shocks. This filled carrier overcomes the problems of the resin-filled carrier described in the above Japanese Patent Laid-Open No. 11-295933.
Further, Japanese Patent Laid-Open No. 2007-57943 discloses a carrier for an electrophotographic developer which is a resin-filled ferrite carrier filled with a resin in the voids of a porous ferrite core material which are continuous from the surface through to the interior, and the carrier has a plurality of three-dimensional layer structures in which a resin layer and a ferrite layer are alternately present. In the working examples of Japanese Patent Laid-Open No. 2007-57943, an example is described wherein 12 to 20 parts by weight of a condensation-crosslinking silicone resin are filled per 100 parts by weight of ferrite core material.
If such a large amount of resin is filled into a porous ferrite core material, some of the resin cannot be filled. This resin is present without closely adhering to the core material, so that there is the problem that the frictional charge with the toner is hindered.
Further, in some cases the floating resin microparticles move onto the electrostatic latent image, leading to image defects such as white spots. In addition, the amount of such floating resin microparticles is different each time the resin-filled carrier is produced, leading to variation in developer characteristics, which dramatically decreases production stability.
Regarding the resin which is filled into or coated onto the carrier core material, and the coated amount, for example, Japanese Patent Laid-Open No. 3-229271 discloses a carrier for an electrophotographic developer which is produced by forming uneven portions on the surface of carrier core particles having a void surface area along a cross-section which includes the major axis of less than 10% by corroding with an acid or alkali, and coating the surface with a resin. Comparative example 2 of Japanese Patent Laid-Open No. 3-229271 describes a core material composed of ferrite particles having a void surface area along a cross-section which includes the major axis of 17.8%, a specific surface area of 915 cm2/g and an average particle size of 95 μm which were treated in hydrochloric acid solution, and a carrier composed of such core material which was coated with an alkaline resin. As also described in the comparative examples of Japanese Patent Laid-Open No. 3-229271, sufficient charge stability could not be obtained with a carrier that had simply been coated with an alkaline resin. Japanese Patent Laid-Open No. 3-229271 contains no specific disclosure concerning the amount of applied resin coating, and also has no teaching concerning the properties of the applied resin. Therefore, although the reason for the charge properties being unstable is uncertain, it can be considered that if a large amount of resin is coated onto a ferrite core material whose specific surface area does not even at most reach 1,400 cm2/g, there is a large amount of floating resin which is not closely adhered to the core material, which becomes a factor in the lack of charge stability. Further, although the average particle size of the carrier described in Comparative example 2 of Japanese Patent Laid-Open No. 3-229271 is about 95 μm, with a carrier having such a large particle size it is difficult to obtain a charging capability which can cope with the recent trend towards a smaller toner particle size.
Japanese Patent Laid-Open No. 2004-77568 discloses a resin-coated carrier for an electrophotographic developer formed with a resin-coated layer on the surface of the carrier core material, wherein the carrier has, on the surface and in the interior of a porous magnetic body with a weight average particle size of 20 to 45 μm, a high resistance substance whose resistance is higher than that of the porous magnetic body itself, and a resistance Log R when applying 5,000 V of 10.0 Ωcm or more.
In Working example 3 of Japanese Patent Laid-Open No. 2004-77568, an example is described in which the steps of mixing 5 kg of core material, 150 g of methyl methacrylate and 5 kg of toluene and then spray drying the mixture are repeated twice, followed by forming a coat of about 0.5 μm with a silicone resin. Specifically, the carrier described in Japanese Patent Laid-Open No. 2004-77568 is such that a resin treatment of at most 6% by weight is carried out on the porous magnetic body particles. With such an amount of resin, it is difficult to achieve a lower specific weight, which makes it difficult to stabilize the charge properties and attain a longer life. Further, the coating resin described in this publication (silicone resin SR-2411) does not have a softening point, so that the occurrence of floating resin cannot be prevented by a method such as that described below in the present invention, and is thus unsuitable for stabilization of charge amount.
In the above-described Japanese Patent Laid-Open Nos. 11-295933, 2006-337579, 2007-57943, 3-229271 and 2004-77568, various types of resin such as those described above are disclosed as examples of the resin for filling or coating. However, there is no disclosure in Japanese Patent Laid-Open Nos. 11-295933, 2006-337579, 2007-57943, 3-229271 or 2004-77568 concerning the softening temperature or curing temperature of the used resin, and merely describe which resins may be used.
For example, in Japanese Patent Laid-Open No. 2006-337579, the condensation-crosslinkable silicone resin SR-2411 (manufactured by Dow Corning Toray Co., Ltd.) and a thermoplastic acrylic resin (manufactured by Mitsubishi Rayon Co., Ltd.) are used. With such resins, when a large amount of resin is filled, a large amount of floating resin that is not closely adhered to the core material may occur, which is believed to be a factor in the lack of charge stability.
In addition, there have been many methods proposed for the filling or coating of the resin into/onto the carrier core material. Japanese Patent Laid-Open No. 5-100492 discloses a carrier for developing an electrostatic charge image produced by mixing magnetic core particles with a coating resin in a dry state and then heating, melting and cooling the mixture, wherein a ferrite magnetic body having a specific surface area of 100 to 1,000 cm2/g is used and the surface coating ratio from the resin is set at 90% or more.
Japanese Patent Laid-Open No. 5-100492 recites in paragraph [0007] that “when producing by a resin coating method without using a solvent, not only is it impossible to obtain a uniform coating, but the interior of the coated portion contains a large amount of voids, whereby film strength is dramatically reduced. Further, if the surface is overly smooth, the anchor effect between the resin coating and the core particles is insufficient, which causes adhesion to deteriorate, whereby there is the problem that the occurrence of floating resin increases during production.” While it is true that resin adhesion can be increased by mixing a ferrite core material which is somewhat uneven with a coating resin in a dry state and then heating, melting and cooling the mixture, a ferrite magnetic body having a specific surface area of at most about 1,000 cm2/g like that described in Japanese Patent Laid-Open No. 5-100492 has few voids, which makes it difficult to achieve a lower specific gravity of the carrier by making the resin permeate into the interior of the magnetic body.
Further, Japanese Patent Laid-Open No. 5-100492 lists various resins which can be used as the resin, and describes that as the carrier particle size, a broad range of 20 to 200 μm can be used. However, the working examples only contain examples of a fluorine resin and a St-MMA resin and particle size of 80 μm. Taking this into consideration, it may be judged that there is no technical suggestion in Japanese Patent Laid-Open No. 5-100492 that a carrier having a small particle size and a low specific gravity can be obtained by filling the void portions of porous ferrite with a resin having specific heat properties.
Japanese Patent Laid-Open No. 5-173371 discloses a carrier for developing an electrostatic charge image coated with core particles, wherein the coating resin contains a methylphenyl silicone polymer having a softening point of 50° C. or above and an absorbance ratio of methyl groups to phenyl groups measured by an IR spectrophotometer in the range of 0.6 to 3.0. This publication also discloses a method for producing a carrier for developing an electrostatic charge image by mixing the coating resin and core particles in a dry state, then heating to melt the coating resin and coating the core particles.
Japanese Patent Laid-Open No. 5-173371 describes coating a specific resin, in which the proper blending amount of the coating resin is about 0.3 to 10% by weight, and preferably 0.5 to 3% by weight. Further, in the working examples the resin amount is at most about 2% by weight. Japanese Patent Laid-Open No. 5-173371 also describes that the ratio of methyl groups to phenyl groups is preferably within a specific range. The reason for this is described as being that if a silicone polymer is used having a ratio of methyl groups to phenyl groups of less than 0.6 but a softening point of 50° C. or above, crosslinking by residual OH groups tends to proceed, it is difficult to obtain a uniform coat by a heating-melting-coating method and peeling tends to occur, and that if a silicone polymer is used having the above-described ratio of 0.6 or more but a softening point of less than 50° C., polymerization tends to be insufficient, a large number of low-molecular weight polymers are included and agglomeration during production or carrier agglomeration after coating tends to occur.
As can be understood from this, Japanese Patent Laid-Open No. 5-173371 merely discloses a resin-coated carrier, and contains no suggestion of the resin-filled carrier like that of the present invention.
Japanese Patent Laid-Open No. 2002-91091 is directed to providing a carrier which can realize high durability and high productivity even while being a resin-coated carrier which can be manufactured at a low cost with high safeness for the environment, and discloses a carrier having on a carrier core material a polyoxyalkylene-modified polyorganosilsesquioxane resin-coated layer which has an amino group, a polyoxyalkylene group and alkoxy group in one molecule.
Further, the proper blending amount of the coating resin is described as being about 0.3 to 10% by mass, preferably 0.05% by mass or more, more preferably 0.1 to 10% by mass and still more preferably 0.2 to 5% by mass. However, Japanese Patent Laid-Open No. 2002-91091 is merely directed to coating on a surface. If the above-described resin is used with such method, sufficient durability cannot be obtained because of the high specific gravity of the resin.
Thus, there is a need for a resin-filled carrier which, while maintaining the advantages of the above-described resin-filled carriers, has stable charging properties when used as a developer, is free from image defects such as white spots and has good production stability.
Accordingly, it is an object of the present invention to provide a resin-filled ferrite carrier for an electrophotographic developer which, while maintaining the advantages of a resin-filled carrier, has stable charging properties when used as a developer, is free from image defects such as white spots and has good production stability, a production method thereof and an electrophotographic developer using this ferrite carrier.